The manufacture of xylene using transalkylation processes utilize one or more catalysts to convert feed streams containing benzene and/or toluene (collectively, C7− aromatic hydrocarbons) and feed streams containing heavy aromatics, i.e., C9+ aromatic hydrocarbons, into a xylene-containing product stream. The demand for xylenes, particularly paraxylene, has increased in proportion to the increase in demand for polyester fibers and film. Supplying the ever-increasing demand has required solving many problems in the production of paraxylene by transalkylation, such as discussed in U.S. Pat. Nos. 5,030,787; 5,763,720; 5,942,651; 6,893,624; 7,148,391; 7,439,204; 7,553,791; 7,663,010; 8,071,828; 8,163,966; 8,183,424; U.S. Patent Publication Nos. 2010-0298117 and 2012-0024755; U.S. patent application Ser. No. 13/811,403; and U.S. Provisional Patent Application Nos. 61/418,212 and 61/496,262. The value of the product is so great that these processes still merit improvement and there is constant research in this area.
A transalkylation system utilizing a single catalyst for transalkylation is illustrated schematically in FIG. 1C, wherein catalyst F is contacted in reactor 120 with feedstream 100 comprising the C7− aromatic hydrocarbons and C9+ aromatic hydrocarbons to produce product 110 comprising xylenes. In a specific example, the catalyst F is ZSM-12 comprising a hydrogenation component and a support, such as Al2O3. The hydrogenation component may be at least one metal or compound thereof, from Groups 6-12 of the Periodic Table.
A dual catalyst system for transalkylation is illustrated schematically in FIG. 1B. In FIG. 1B a feedstream 101 comprising C7− aromatic hydrocarbon and C9+ aromatic hydrocarbon first contacts catalyst D in reactor 121 and then the product of the contact with catalyst D contacts catalyst E, providing product 111 comprising xylenes. One example of a two-catalyst system D followed by E, respectively, is ZSM-12 comprising a hydrogenation component and a support, followed by a catalyst comprising ZSM-5 without a hydrogenation component. In this case the presence of the second catalyst is found to improve purity by cracking certain undesired co-boilers that make separation of the desired product(s) more difficult. A second example of a two-catalyst system D followed by E, respectively, is ZSM-5 comprising a hydrogenation component followed by ZSM-12 comprising a hydrogenation component. In this case the ZSM-5 and hydrogenation component facilitates de-alkylation of C10+ aromatic hydrocarbons to enhance recovery of desired products and/or reduce aging of the downstream ZSM-12 component.
A three-catalyst system for transalkylation is illustrated schematically in FIG. 1A. In FIG. 1A a feedstream 102 comprising C7− aromatic hydrocarbons and C9+ aromatic hydrocarbons contacts catalysts A, then catalyst B, then catalyst C, to produce product 112 comprising xylenes. One example of such a three-catalyst system is ZSM-5 with a hydrogenation component, followed by ZSM-12 comprising a hydrogenation component, and then ZSM-5 without a hydrogenation component, where again the ZSM-5 as the third component cracks certain undesired co-boilers to enhance purity of the final product.
A typical feed to such process can be any conventional C8+ aromatic hydrocarbon feed available in a petroleum or petrochemical refinery, such as a catalytic reformate, FCC or TCC naphtha, or a xylene isomerizate from which heptanes and lighter components have been removed. The feed is initially passed through a xylenes fractionation column or columns to remove the C8 aromatic components from the feed and leave a C9+ aromatic hydrocarbon-rich fraction which can then be fed to a transalkylation reactor for reaction with benzene or toluene in the presence of a transalkylation catalyst system, such as described above, to produce lighter aromatic products, primarily benzene, toluene, and xylenes (collectively, “BTX”). These components can then be separated by methods well-known in the art, and all or a portion of the benzene and toluene can be recycled through the transalkylation system.
In the case of the use of multiple catalysts, the first, second and optional third catalyst beds may be located in separate reactors, such as illustrated schematically in FIG. 1D. FIG. 1D illustrates an alternative to FIG. 1A. In FIG. 1D, feedstream 103 contacts catalyst A in first reactor 123, then catalyst B in second reactor 124, and finally catalyst C in third reactor 125, providing product 112 comprising xylenes.
In the case of multiple catalysts in a single reactor, the catalysts will typically be separated from each other by spacers or by inert materials, such as, alumina balls or sand. Alternatively, the first and second catalyst beds could be located in one reactor and the third catalyst bed (when present) is located in a different reactor. As a further alternative, the first catalyst bed could be located in one reactor and the second and third catalyst beds (when present) are located in a different reactor. In all situations illustrated, the separate catalysts such as A, B, and C in both FIG. 1A and FIG. 1D are not mixed and the hydrocarbon feedstocks and hydrogen are arranged to contact the first catalyst bed prior to contacting the second catalyst bed. Similarly, if the third catalyst bed is present, the hydrocarbon feedstocks and hydrogen are arranged to contact the second catalyst bed prior to contacting the third catalyst bed. However, numerous systems are known wherein at least two different catalysts are mixed together in a single bed. Likewise, while the FIGS. 1A-1D show axial flow reactors, it will be appreciated by one of skill in the art that radial flow reactors could also be used, alone or in combination with axial flow reactors.
U.S. Pat. No. 7,629,499 describes a process for transalkylation comprising: (a) introduction of alkyl-aromatic feedstock at the inlet of a first reaction zone to contact a first zeolite catalyst to obtain an effluent; (b) introduction of at least a portion of the effluent from the first reaction zone and a feedstock that contains benzene and toluene to the inlet of a second reaction zone that contains a second zeolite catalyst to obtain a second effluent; and (c) separation of at least a portion of the second effluent. An improvement in xylene yield is said to be achieved by this method. Hydrogen is introduced at each inlet.
Catalyst life and extent of aromatic ring loss reactions across the reactor are two important performance characteristics for transalkylation catalysts and catalyst systems. Typically it is ideal to maximize catalyst life, while at the same time reducing the extent of aromatic ring loss reactions (which reduces yield), and generally increasing the desired products (paraxylene in particular) and decreased the undesired products (such as benzene co-boilers).
The present invention uses two or more separate staged feed locations to the transalkylation reactor system in order to improve yields (including reduced ring loss reaction) and to increase catalyst life and attenuates the introduction of hydrogen through the two or more separate staged feed locations to effect improved results.